The present invention is generally related to the field of scanners and, more particularly, is related to the illumination of light emitting diodes in a contact image sensor in a scanner.
Conventional scanners that employ contact image sensors typically employ light emitting diodes (LED) to illuminate the subject that is scanned. Such a subject may be, for example, a document. The light that reflects from the subject is sensed by a multitude of sensors in the contact image sensor that generates corresponding signals that are representative of the scanned subject as is generally known by those with ordinary skill in the art.
To illuminate the subject to be scanned, a light pipe is typically employed to distribute light generated by a single LED across the entire subject to be scanned, thereby providing light that can be sensed by the entire contact image sensor. For color scanners, typically three different LED""s of different colors are used such as a red, green, and blue. These different color LED""s are switched on at different times to obtain three respective exposures of each dot or pixel on the subject scanned.
In one conventional configuration, each of the LED""s is coupled to a power supply with a series resistor. In this configuration, the combination of the power supply voltage, the resistance of the series resistor, and the forward voltage of the specific LED determine the current through the LED which, in turn, determines the light output from the LED. Unfortunately, the forward voltage of each of the LED""s and the resistance of the resistor often vary due to production process variation and other factors. Also, the power supply voltage may vary due to environmental conditions such as temperature, etc. Due to the combination of the variations noted above, the resulting current through each of the LED""s generally varies greatly, thereby resulting in significant variation in the light output generated by each of the LED""s. In addition, variation in other aspects of the image scanner system such as the sensitivity of the contact image sensors results in corresponding variation of the required amount of light that should be generated by each of the LED""s. For example, the sensitivity of the contact image sensors may vary over time with repeated use.
In another conventional design, a constant current source is employed with each of the LED""s to ensure a fixed current flows therethrough. However, this design is subject to the problem of the variation in the manufacturing of the LED""s. In particular, for a number of LED""s created in a single batch, a distribution of light output results among the LED""s in the batch. That is to say, the same current flowing through each LED in a batch will result in a different light output for each LED. In addition, such a constant current source does not address the variation in the other aspects of the image scanner system that may require a different amount of light than that which is produced by the LED""s that are driven by a constant current source.
As a consequence of the foregoing, LED""s in conventional image scanner systems generate less than optimum light outputs based upon the needs of the contact image sensors, thereby negatively impacting the quality of resulting scanned images.
In light of the foregoing, the present invention provides for a circuit and method for generating light to illuminate a subject such as a print medium for scanning using, for example, a contact image sensor. According to one embodiment, the circuit includes a light emitting diode and a variable current control circuit coupled to the light emitting diode. The variable current control circuit is configured to establish a current through the light emitting diode, the magnitude of the current being variable. The variable current control circuit includes a programmable current sink. Alternatively, the variable current control circuit may also include an offset current sink. The programmable current sink and the offset current sink (if included) are employed to establish the variable current through the light emitting diode.
The variable current control circuit further includes a reference current circuit generating a reference current based upon a band gap voltage reference. Both the programmable current sink and the offset current sink (if included) are referenced from the reference current. The use of the band gap voltage reference allows the creation of the reference current with little susceptibility to fluctuation due to changes in temperature or other environmental factors. The circuit further comprises a current control register that is coupled to a current control input of the programmable current sink. The magnitude of the current established through the LED varies with reference to a current control value stored in the current control register.
The present invention may also be viewed as a method for generating light. The present method comprises the steps of: generating a current through a light emitting diode to create a light output, and, varying a magnitude of the current, thereby causing a corresponding variation in the light output. In the present method, the step of varying the magnitude of the current may further comprise the step of varying the magnitude of the current among a number of discrete current levels. Also, the step of varying the magnitude of the current may further comprise the step of varying the magnitude of the current with a programmable current sink. In addition, the step of generating the current through the light emitting diode to create the light output may further comprise the step of generating the current with an offset current sink.
In order to reference the programmable current sink, the step of varying the magnitude of the current with the programmable current sink further comprises the step of generating a reference current to reference the programmable current sink. Alternatively, the step of generating the current with the offset current sink may further comprise the step of generating a reference current to reference the offset current sink.
The step of generating a reference current to reference the programmable current sink or the offset current sink may include, for example, the step of generating the reference current based upon a band gap voltage reference. This is done, for example, to ensure that the reference current generated is constant even in the presence of temperature fluctuation or other environmental changes.
Other features and advantages of the present invention will become apparent to a person with ordinary skill in the art in view of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.